The present invention generally relates to apparatus for providing a reference laser beam and, more particularly, to laser beam projecting apparatus having a generally cylindrical continuous lighthouse structure and a compensating lens. The compensating lens corrects for divergence of the reference beam before it emerges from the projecting apparatus through the lighthouse structure such that the projected reference beam will have a level of intensity and range of detectability substantially equivalent to a beam transmitted undeviated through a planar window having parallel surfaces.
Laser beam systems have been employed in numerous surveying and construction applications. In one such system disclosed in U.S. Pat. No. 4,062,634, which issued Dec. 13, 1977 to Rando et al and is assigned to the assignee of the present invention, a laser beam projecting apparatus provides a rotating laser beam which establishes a reference plane. Typically, the rotating laser beam is used to provide a continuous plane of light that creates a constant horizontal bench mark of elevation over an entire work area. Also, the system employs one or more laser beam detectors, placed at considerable distances from the projecting apparatus, for intercepting the rotating laser beam and determining the proper elevation at selected points throughout the work area.
In the laser beam projecting apparatus, the generally horizontal rotating reference laser beam is produced by projecting the beam generally upward and then deflecting it ninety degrees within a pentaprism or penta-mirror assembly. The pentaprism assembly is rotated about a vertical axis within the projecting apparatus to cause the horizontal beam to rotate and define the reference plane.
To adapt the reference laser beam projecting apparatus for use in surveying and construction applications under a variety of field conditions, the projecting apparatus is provided with a waterproof housing having an upper transparent lighthouse assembly within which the pentaprism assembly rotates and through which the laser beam is projected. The lighthouse assembly typically includes four upstanding flat transparent panes of glass of high optical quality which are joined together at their side edge surfaces so as to form a square in cross section. The lower edges of the panes are mounted within recessed seats formed in an upper rim of the housing, while the upper edges of the panes are mounted within recessed seats formed within a top casing of the housing. The upper and lower edges of the panes are sealed by gaskets. Four posts are threadably mounted between the top casting and upper rim for applying sufficient axial forces to pull them together and lock the panes into their seats. The four posts are aligned radially outwardly of corner junctures formed between the panes, while the panes have forty-five degree bevel side edge surfaces which interface to form the corner junctures and which are sealed together by a suitable adhesive.
While providing precise beam positioning, the lighthouse assembly described above is made up of a number of components which require careful attention and precise aligning and positioning in order to assemble it. Specifically, the panes through which the laser beam passes must be of extremely high optical quality. This requires extensive, careful individual inspection of a large number of similar panes until ones of suitable optical quality are found. Then, the individual panes must be carefully ground to precise dimensions to provide the bevel side edge surfaces which form the corner junctures of the lighthouse. Finally, the panes must be positioned very precisely relative to one another and adhered together at their bevel side edge surfaces. It is, therefore, seen that a need exists for improvement of the lighthouse assembly construction which will eliminate some of the above-outlined difficulties encountered in selecting and fitting the components of the lighthouse assembly together.